This invention relates to birefringent dielectric multilayer reflecting films and laminate articles made therefrom.
A conventional automotive safety glazing is formed from a laminate made of two rigid layers, typically glass, and an anti-lacerative mechanical energy absorbing interlayer of plasticized polyvinyl butyral (PVB). The glazing is prepared by placing the PVB layer between glass sheets, eliminating air from the engaging surfaces, and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB and glass into an optically clear structure. The glazing may then be used in the windows, windshields or rear glass of a motor vehicle.
The laminate may also include at least one functional layer engineered to enhance the performance of the vehicle window. One important functional layer reduces entry of infrared radiation into the vehicle cabin. Infrared rejecting functional layers are typically made of metallized or dyed polymer film constructions that reflect or absorb unwanted solar radiation. When used in a windshield, the composite laminate structure should transmit at least about 70% of the light in the wavelength region sensitive to the human eye, typically from about 380 to about 700 nanometers (nm), and reject solar radiation outside the visible portion of the spectrum. When used in other glazing structures, such a side or rear windows, there are typically no limits on the level of visible transmission.
Referring to FIG. 1A, a pre-laminate structure 10 is shown that may be bonded to one or more glass sheets to make a vehicular safety glazing laminate. The pre-laminate 10 includes a reflective functional layer 12 that includes a polymer layer 14 and a metallized layer 16. The functional layer 12 is bonded on at least one side to at least one layer 18 of PVB. Optionally, the functional layer 12 may be bonded to a second layer 20 of PVB. One or the other or both of the PVB layers 18, 20 may include additional performance enhancing layers. For example, the PVB layer 20 may optionally include a shade band layer 22.
Referring to FIG. 1B, the pre-laminate structure 10 may be matched with at least one, preferably two, sheets of glass 30, 32 to form a safety glazing laminate 34. To bond the pre-laminate 10 to the glass sheets 30, 32, the pre-laminate 10 and the sheets 30, 32 are placed together. The laminate 34 is heated to cause the PVB layers 18, 20 and the functional layer 12 to conform to the contours of the glass sheets 30, 32. The laminate 34 may be assembled by one of three different methods. Two of the methods use a vacuum de-airing process where a flexible band, ring or bag is placed around the edge of the laminate and connected to a vacuum system while the laminate is pre-heated to generate a temporary bonding between the glass and PVB. Another method uses a pressure roller device, referred to herein as a nip roller, which applies pressure to the laminate to de-air and to promote bonding between the layers. Compared to the vacuum de-airing processes, the nip roll process requires fewer manual process steps and allows the laminates to be assembled more quickly. For at least these reasons, the nip roll process is a preferred method for many automotive glazing manufacturers.
To enhance vehicle aerodynamics and improve outward visibility, vehicular window shapes are not planar, and increasingly include severe angles and complex curves. When the laminate 10 is placed between complex curved glass sheets and laminated with a nip roll process, or heated to bond the PVB to the glass. The functional layer 12 cannot perfectly conform to the complex curvatures, especially when the glass sheets are large. Wrinkles, folds and pleats can form in the functional layer, and, when the functional layer is metallized, cracks can form in the metallized layer 16 during nip rolling, which creates an optical defect in the safety glazing. As a result, only small size laminates with no curvature or a small one-dimensional curvature can currently be manufactured using a nip roll process.
Birefringent, non-metallic films made from alternating layers of dielectric materials, preferably polymers with differing indices of refraction, may be engineered to reflect or absorb a desired amount of light in a spectral region of interest while transmitting sufficient visible light in the visible region of the spectrum to be substantially transparent. These birefringent dielectric optical films preferably include alternating layers of a first material having a first index of refraction and a second material having a second index of refraction that is different from the first index of refraction.
The film is preferably a multilayer stack of polymer layers with a Brewster angle (the angle at which reflectance of p polarized light goes to zero) that is very large or nonexistent. The film is made into a multilayer mirror whose reflectivity for p polarized light decreases slowly with angle of incidence, is independent of angle of incidence, or increases with angle of incidence away from the normal. This multilayered film has high reflectivity (for both s and p polarized light) for any incident direction.
The reflectance characteristics of the multilayer film are determined by the in-plane indices of refraction for the layered structure. In particular, reflectivity depends upon the relationship between the indices of refraction of each material in the x, y, and z directions (nx, ny, nz). The film of the invention is preferably constructed using at least one uniaxially birefringent material, in which two indices (typically along the x and y axes, or nx and ny) are approximately equal, and different from the third index (typically along the z axis, or n2). The x and y axes are defined as the in-plane axes, in that they represent the plane of a given layer within the multilayer film, and the respective indices nx and ny are referred to as the in-plane indices.
One method of creating a uniaxially birefringent system is to biaxially orient (stretch along two axes) the multilayer polymeric film. If the adjoining layers have different stress-induced birefringence, biaxial orientation of the multilayer film results in differences between refractive indices of adjoining layers for planes parallel to both axes, resulting in the reflection of light of both planes of polarization. A uniaxially birefringent material can have either positive or negative uniaxial birefringence. Positive uniaxial birefringence occurs when the index of refraction in the z direction (nz) is greater than the in-plane indices (nx and ny). Negative uniaxial birefringence occurs when the index of refraction in the z direction (nz) is less than the in-plane indices (nx and ny).
If n1z, is selected to match n2x=n2y=n2z and the multilayer film is biaxially oriented, there is no Brewster""s angle for p-polarized light and thus there is constant reflectivity for all angles of incidence. Multilayer films that are oriented in two mutually perpendicular in-plane axes are capable of reflecting an extraordinarily high percentage of incident light depending of the number of layers, f-ratio, indices of refraction, etc., and are highly efficient mirrors.
A second factor that determines the reflectance characteristics of the multilayer film is the thickness of the layers in the film stack. Adjacent pairs of layers (one having a high index of refraction, and the other a low index) preferably have a total optical thickness that is xc2xd of the wavelength of the light to be reflected. For a two-component system, to achieve maximum reflectivity the individual layers of a multilayer polymeric film have an optical thickness that is xc2xc of the wavelength of the light to be reflected, although other ratios of the optical thicknesses within the layer pairs may be chosen for other reasons. Optical thickness is defined as the in-plane refractive index of a material multiplied by the actual thickness of the material, and all actual thicknesses discussed herein are measured after any orientation or other processing.
For example, by selecting the layer thicknesses to reflect near infrared light, and positioning the reflective bandedge within the infrared region such that even at grazing angles of incidence the reflectance band does not shift into the visible region of the spectrum, an infrared mirror can be made that is transparent in the visible region of the spectrum, even at high angles of incidence. The infrared (IR) reflecting films described in U.S. Pat. Nos. 5,882,774 and 6,049,419, each incorporated herein by reference, control the amount of solar energy that pass through them, preferably without significantly decreasing the intensity or changing the color of light sensed by the human eye at any angle. The materials in the layers, the thicknesses of the layers, and the indices of refraction of the layers are selected to reflect infrared radiation within the wavelength range of about 700 nm to about 2000 nm, while transmitting visible light. The film has an average reflectivity of at least 50% over a band at least 100 nm wide in the infrared region of the spectrum. These films have been applied to substantially flat substrates to form laminates. However, when applied to a non-planar substrate, wrinkles form in the film, so the films have not been used in laminates with severely curved or compound curved substrates. The wrinkles are a particular problem in laminates that should be substantially optically clear, such as, for example, laminates intended for use in vehicular windshields.
In a first aspect, the invention is a process for making a film. The process includes providing a birefringent dielectric multilayer film that reflects at least 50% of light in a band at least 100 nm wide in a wavelength region of interest. The film is heat set to render the film capable of shrinking to conform without substantial wrinkling to a substrate having a compound curvature. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In a second aspect, the invention is a process for making a film. The process includes providing a birefringent dielectric multilayer film that reflects at least 50% of light in a band at least 100 nm wide in a wavelength region of interest. The film is heat set at a temperature sufficient to enable the film to shrink at least about 0.4% in both in-plane directions upon heating. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In a third aspect, the invention is a birefringent dielectric multilayer film that reflects at least 50% of light in a band at least 100 nm wide in a wavelength region of interest. The film is heat set at a temperature sufficient to render the film capable of shrinking to conform without substantial wrinkling to a substrate having a compound curvature. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In a fourth aspect, the invention is a birefringent dielectric multilayer film that reflects at least 50% of light in a band at least 100 nm wide in a wavelength region of interest. The film is heat set at a temperature sufficient to enable the film to shrink at least about 0.4% in both in-plane directions upon heating. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In a fifth aspect, the invention is a process for making a laminate article. The process includes assembling a laminate with the following layers: a first non-planar layer of a glazing material such as glass, a first energy absorbing layer, a film layer, a second energy absorbing layer and a second non-planar layer of glazing material. The film layer is a birefringent dielectric multilayer film that reflects at least 50% of light in a band at least 100 nm wide in a wavelength region of interest. The laminate is heated to remove residual air between the layers, and bond the layers such that the energy absorbing layers and the film layer conform to the shape of the non-planar glazing layers. The laminate is further heated and pressure is applied to the laminate to bond the layers together and form an optical structure, and the structure is cooled, wherein the structure exhibits substantially no wrinkling in the film layer. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In a sixth aspect, the invention is a process for nip roll laminating a glazing article, including assembling a laminate with the following layers: a first non-planar layer of a glazing material, a first energy absorbing layer, a non-metallized film layer, a second energy absorbing layer and a second non-planar layer of a glazing material. The film layer reflects at least 50% of light in a band at least 100 nm wide in a wavelength region from about 700 to about 2000 nm. The laminate is heated to remove residual air between the layers, and the layers are bonded with a nip roller. The energy absorbing layers and the film layer conform to the shape of the non-planar glass layers without substantial cracking and or creasing.
In a seventh aspect, the invention is a pre-laminate including at least one layer of an energy absorbing material and a layer of a film. The film layer is a birefringent dielectric multilayer film that reflects light at least 50% in a band at least 100 nm wide over a wavelength region of interest. The film is heat set at a temperature sufficient to render the film capable of shrinking to conform without substantial wrinkling to a substrate with a compound curvature. Preferably, the wavelength region of interest is from about 700 nm to about 2000 nm.
In an eighth aspect, the invention is an optically clear laminate article including the following layers: a first non-planar layer of glass, a first energy absorbing layer of PVB, a film layer, a second energy absorbing layer of PVB and a second non-planar layer of glass. The film layer is a birefringent dielectric multilayer film that reflects that reflects at least 50% of light in a band at least 100 nm wide positioned between wavelengths from about 700 nm to about 2000 nm. Preferably, the laminate article is a windshield.
In a ninth aspect, the invention is a vehicle with a glass article. The article is an optically clear laminate including the following layers: a first non-planar layer of glass, a first energy absorbing layer of PVB, a film layer, a second energy absorbing layer of PVB and a second non-planar layer of glass. The film layer is a birefringent dielectric multilayer film that reflects that reflects at least 50% of light in a band at least 100 nm wide positioned between wavelengths from about 700 nm to about 2000 nm.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.